The present invention relates to a plastic sorting method and a plastic sorting apparatus for sorting, according to type, crushed waste composed of mixed plastic pieces of a plurality of types.
In recent years, recycling of waste has proceeded tremendously. Among different plastics that are consumed as raw material for plastic articles, polyvinyl chloride resins (hereinafter referred to as xe2x80x9cPVCxe2x80x9d), polyethylene resins (hereinafter referred to as xe2x80x9cPExe2x80x9d), polypropylene resins (hereinafter referred to as xe2x80x9cPPxe2x80x9d), polystyrene resins (hereinafter referred to as xe2x80x9cPSxe2x80x9d), and methacrylic resins [acrylic resins] (hereinafter referred to as xe2x80x9cPMMAxe2x80x9d) account for about 80%, and these types of resins account for almost all the plastic waste that is recovered. Also polyethyleneterephthalate resin (hereinafter referred to as xe2x80x9cPETxe2x80x9d), which is used in so-called PET bottles, is increasingly collected individually. When these resins are recycled, it is indispensable to classify (sort) them by resin type.
Conventionally, an apparatus for sorting plastic pieces, using charging properties of the plastic pieces has been provide as a technology for sorting crushed plastic pieces. Such a conventional plastic sorting apparatus is explained with reference to FIG. 1 showing a general configuration of a plastic sorting apparatus in accordance with a first embodiment of the present invention.
In this plastic sorting apparatus S, plastic pieces 1 of various different types of resins, or crushed waste to be sorted, are introduced in a mixed state into a hopper 2. A mixture of resin plastic pieces 1 of different types is supplied from an outlet port of the hopper 2 into a frictional charging device 3, the plastic pieces 1 are frictionally charged by agitating them in this frictional charging device 3, and then they are distributed over an upper surface of a metal drum electrode 5. The metal drum electrode 5 is rotatable in a predetermined direction around a horizontal axis and is grounded.
An arcuate plate-shaped high-voltage electrode 6 is arranged in a rotational direction diagonally above the metal drum electrode 5. A pole, for example a negative pole, of a high-voltage power source 7 is connected to the high-voltage electrode 6, whereas a positive pole of the high-voltage power source 7 is grounded. Being thus connected, the metal drum electrode 5 forms a rotating grounded electrode, and an electrostatic field for sorting is generated between the high-voltage electrode 6 and the metal drum electrode 5.
Below the metal drum electrode 5, a first separation container 8 and a second separation container 9, which are open upward, are arranged sequentially on the upstream side in the rotational direction. A brush 10 for scraping off plastic pieces 1 that adhere to the peripheral surface of the metal drum electrode 5 is disposed on the outer periphery of the metal drum electrode 5.
The following explains how this configuration operates. Various types of plastic pieces 1 are introduced-in a mixed state into the frictional charging device 3 from the hopper 2. Inside the cylinder of the frictional charging device 3, different types of plastic pieces 1 are agitated and rubbed against one another so that frictional charging of the plastic pieces is effected depending on a polarity and amount of electric charge for each plastic piece. Then, the charged plastic pieces 1 are scattered over the metal drum electrode 5. The plastic pieces 1 that have been charged with a negative charge in the frictional charging device 3 are repelled by the high-voltage electrode 6 and attracted by the metal drum electrode 5, and fall into the second separation container 9 due to the rotation of the metal drum electrode 5 or fall into the second separation container 9 when they are scraped off the surface of the metal drum electrode 5 with the brush 10. On the other hand, the plastic pieces 1 that have been charged with a positive charge, that is, a charge that is opposite to that of the metal drum electrode 5, adhere to the surface of the metal drum electrode 5, and fall into the first separation container 8 due to the centrifugal forces of the metal drum electrode 5.
The conventional frictional charging device 3 agitates and frictionally charges the plastic pieces 1 comprised of a plurality of different type resin plastics that have been introduced into the hopper 2, but when there is a large difference between the different type plastic pieces 1 in amount (that is, a difference in surface area), the plastic pieces 1 may not be imparted, during agitation, with necessary polarity and amount of charge (charge voltage) for electrostatical separation. In such a case, it was not possible to sufficiently sort the plastic pieces 1 with the metal drum electrode 5 or the high-voltage electrode 6 using the charges on the plastic pieces 1.
It is thus an object of the present invention to provide a plastic sorting method and a plastic sorting apparatus, with which the above-described problems can be solved.
To solve these problems, the present invention provides an apparatus and a method, wherein, if the proportion of the specified plastic pieces to be recovered is very low, then the frictional charging is performed after adding more of the specified plastic pieces, or the same type of plastic pieces, or plastic pieces located in the middle of the charging order of the mixed plastic pieces, or plastic pieces located on the positive or the negative side in a predetermined charging order as an auxiliary frictional charging material, and the added auxiliary frictional charging material remains in the frictional charging device and is used repeatedly.
With this configuration, even if there is relatively little auxiliary frictional charging material, the charging can be performed with an adequate polarity and amount of charge, so that the specified plastic pieces can be separated and recovered with high purity and recovery rate. Furthermore, it is economic, because the auxiliary frictional charging material can be used several times.
In another aspect of the present invention, if the proportion of the specified plastic pieces to be recovered is very low, the same effect can be attained by a special design of the frictional charging device, instead of adding the specified plastic as the auxiliary frictional charging material. That is to say, the frictional charging device is configured of a substantially cylindrical container, a plurality of agitating rod members that are attached in radial fashion to a rotation shaft rotating about the center of the container, and a plurality of auxiliary agitation members that are fixed to an inner container and arranged between the agitating rod members. The plastic sorting method and the plastic sorting apparatus are also characterized in that the surface area of the agitating rod members and the auxiliary agitation members is no less than 10% of the total surface area of the plastic pieces that are introduced into the frictional charging device at one time.
With this configuration, the agitating rod members and the auxiliary agitation members function as auxiliary frictional charging material and each type plastic piece is charged with a predetermined polarity and amount of charge, so that plastic pieces of intended types can be separated and recovered with high purity and recovery rate.